lsayre wrote:As late as the 1970's a NG, propane, or oil fired appliance was doing pretty good if it had an output BTU efficiency of 70%. Perhaps I'm looking at this entire argument form the perspective of output efficiency, and everyone else is looking at some other efficiency?

As early as the 1940s there were oil burners that could burn with 14 percent CO2 which few modern burners if any can match. 15 percent is considered perfect. Most burners of that time were smudge pots with CO2 around 8. How the heat from the fire is handled is something else and two different burners in the same furnace or boiler can have dramatically different overall efficiency.

What is the BTU transfer efficiency of these stoves into the home? And where are these in the real world? The reality is that residential oil heating appliances from this era have output efficiencies as low as 50% in the real world, to perhaps 60-65% at best, with much of their potential heat going out the stack. Perhaps you are talking from a lab environment perspective. Enter the real world where people heat their homes with what is available and show us examples of the common people heating their homes in the late 40's with oil burning appliances so efficient that they can't be improved upon today.

Even today if it doesn't condense the flue gases to scavenge the heat that will be lost to the chimney, I can't see how it is going to be better than about 85% to perhaps 87% maximum real world efficient at heat transfer, regardless of the fuel source.

I was referring to the rotary oil burners such as the Timken which were ideal for installation in the old round coal boilers. These burners converted the oil to a vapor or gas much finer than the droplets of a modern burner. They mixed air with the vapor before burning which resulted in a rolling blue flame filling the boiler. A cloud of blue flame.

They are no longer around because they also had bad points. There was no oil pump but instead relied on gravity feed which made it necessary to install an oil tank on its side and raise it high enough to feed taking up more room than a vertical tank. Start up was smoky and to achieve the efficiency it was capable of, adjustment was critical including a very sensitive barometric damper. Because it was so different from the typical burner many service men did not understand it and failed to adjust properly. If the solenoid valve which shut off the oil failed it had the habit of incinerating the motor.

Because so little excess air was needed stack temperatures were very low approaching the minimum needed.

Also consider this that stack temperature should be about 250 to avoid condensation and this is true regardless of the size of the burner. A unit putting out 100,000 BTU running a stack of 250 loses a lower percentage of its heat to the chimney than a unit half its size in output. So size can also be important in overall efficiency. Condensing furnaces and boilers have their own set of problems getting rid of the condensate.